Recently, memory density of a storage medium such as a magnetic disk device, a magneto-optical disk device, and an optical disk device has been improved considerably. As a result, downsizing and increase of memory capacity of the storage devices have become possible, thereby improving the conveniences of such devices.
In such a storage medium having a high density, automatic control referred to as servo control has been used for accurately positioning a head on a targeted track. A signal pattern referred to as a servo pattern to be used for the servo control is written on the storage medium.
The servo pattern includes at least a track number, a sector number, and a servo pattern for demodulating a relative position of the head with respect to a track center. In a storage device, for example a magnetic disk device, a servo-pattern transfer process is adopted for transferring a servo pattern included in a transfer master onto a storage medium by magnetic transfer or nanoimprint lithography.
A foreign substance such as dust or defect such as flaw can be present on the surface of a storage medium. In a servo-pattern transfer process, if a servo pattern is transferred to a portion where a foreign substance or defect is present, the transfer of the servo pattern becomes defective. The head cannot be accurately positioned on a targeted track on a storage medium having a defective servo pattern, which has not been transferred accurately, and therefore a corresponding user data storage area cannot be used.
Further, in the servo-pattern transfer process, it is required not to damage its transfer master. Therefore, it is required to detect a foreign substance or defect on a storage medium, to which a servo pattern is transferred, by using an optical test or the like, in order to remove the foreign substance or to discard a storage medium itself without performing transfer of the servo pattern.
As a method of detecting a foreign substance or defect on a surface of a storage medium, there is a method of detecting a foreign substance or defect based on reflected light or scattered light by irradiating laser beams. There is also a method of detecting a foreign substance or defect on a transfer master, like the method of detecting a foreign substance or defect on the surface of the storage medium.
The position of the foreign substance or defect on the storage medium detected in the above manner is overlapped on a servo pattern of a transfer master by pattern matching, to determine whether the position overlaps on the pattern of the transfer master.
Further, there has been proposed a method that a plurality of servo patterns are prepared on a transfer master on the assumption that a foreign substance or defect is always present, to perform transfer to a storage medium, and a servo pattern having the lowest overlapping degree with the foreign substance or defect on a surface of a storage medium is selected and used.
See the documents such as Japanese Laid-open Patent Publication No. 2001-351231, Japanese Laid-open Patent Publication No. 2003-4428, Japanese Laid-open Patent Publication No. 2002-372501, and International Publication Pamphlet No. WO2005/001817.
However, in the conventional techniques, there is a problem such that there have been more demands to form a finer servo pattern on the storage medium, as the memory density of the storage medium has becomes much higher. As the servo pattern becomes finer, there is a problem of finer foreign substance or defect having a size of several micrometers, for example.
Therefore, the foreign substance or defect cannot be completely removed from the surface of the storage medium. Further, there is a limitation on the size of a foreign substance and the like to be measured by an optical detection device, and it is practically difficult to detect a foreign substance or defect with a resolution of nanometer, which can correspond to a requirement of positioning accuracy with the nanometer level.
Further, if it is assumed that a foreign substance or defect is detected from a surface of a storage medium, and the storage medium having the detected foreign substance or defect is to be discarded, a large amount of storage media needs to be discarded before transfer of the servo pattern, thereby causing production cost increase of the storage device.
In the method in which a plurality of servo patterns are prepared on a transfer master to perform transfer to a storage medium, and a servo pattern having the lowest overlapping degree with a foreign substance or defect on a surface of a storage medium is selected and used, there is the following problem.
That is, this method cannot be applied to a storage medium requiring transfer of a fine pattern, such that a certain pattern is formed also in a data area between the servo patterns, such as a discrete track or a bit patterned media.